The present invention relates to an X-ray intensifying screen made of a material for transforming X-rays into light and of a suitable binding agent therefor.
In X-ray imaging it is customary to employ intensifying screens on one or both sides of the X-ray film. The purpose of the screens is to transform the X-ray quanta into light quanta, for which the X-ray film is most sensitive. The materials used in intensifying screens do not, however, fully transmit the light emitted by them. If the screen is made thick, the intensifying material absorbs the created light. Thus only a thin layer of the intensifying screen can be utilized.
Presently the common intensifying material is a radiation-absorbing and light-emitting material, i.e. luminescent material, which is applied as a thin layer on the surface of a suitable base material. The intensifying screen formed in such a fashion can also be protected with a suitable film or coating.
Although the intensifying screen is an essential component in medical X-ray imaging, two factors make it a weak link in the chain of information transmission which comprises the total image formation process. Firstly, the screens fail to use a large proportion of the available information carrying X-ray quanta by not absorbing the quanta at all. Secondly, the positional information of the absorbed quanta is diminished by the fact that light spreads in the screen through diffusion. This means that the information on the position of the X-ray quantum absorption becomes less precise due to the statistical nature of the diffusion of the finite number of light quanta created. These two factors are strongly interrelated in conventional flat intensifying screens in the sense that improvement in one means deterioration in the other. Furthermore, not even the absorbed quanta are registered equally on the film. Those near the film surface contribute most to the blackening of the film whereas the quanta absorbed at a distance have less influence on the blackening. This phenomenon is most pronounced in thick screens and reduces the average contribution of the absorbed quanta to light output. Information in X-ray imaging is transmitted by X-ray quanta incident on the screen-film system. The information transmission capacity of the system depends on the ability of the system to register the X-rays on the film. The blackening corresponding to each quantum should be equal to ensure maximum information transmission. The maximum amount of blackening is not important in this respect, since the sensitivity of the film can easily be adjusted. To elaborate further, if the variation in light output corresponding to quanta absorbed at different depths is large, the uncertainty in the actual number of quanta giving rise to a given blackening of the film increases. For example, a given blackening could be due to one quantum absorbed at the film surface of the screen or 10 quanta absorbed at the far surface of the screen. This uncertainty, which reduces the information transmission, is most pronounced at the low quantum densities typical of rare earth screens.
An X-ray intensifying screen is known in the art and disclosed in the German Offenlegungschrift-publication No. 2 010 780, wherein the luminescent material is divided into smaller areas and these areas are separated by metal slits. By making these areas sufficiently small, the light quanta diffusion and hence the image quality can be somewhat improved. Despite that, this screen has the same drawbacks as described above. Moreover, the intensifying screen presented in this German publication becomes expensive to manufature.